The CRISPR/Cas12a (cpf1) system was reported to indiscriminately cleave single-stranded DNA after binding with target DNA strands. This usually required the target DNA strands to contain the protospacer-adjacent motif (PAM) sequence of TTTN. Herein, we found Cas12a can also recognize another PAM sequence of UUUN resulting in activation of its ssDNA collateral cleavage effect. To make this finding useful, by combining with LAMP, we first realized CRISPR/Cas12a for directly visualized DNA detection at the single-copy level. By treating with UDG enzyme, we made this system free from residual amplicon contamination, which is a big problem in this field. Thus, an ultrasensitive and anticontaminant DNA detection platform, namely, UDG and LAMP and CRISPR (ULC). This new finding would help us better understand the mechanism of Cas12a and expand its application.